You can do this without problems, but if some of the posts rank before the original content, than you can have a problems. Keep in mind that not every author wants its content copied.
Google ranking or Keyword ranking - if there have some good ranking they will be for very short period.
Pagerank - as I know it comes from the backlinks. Some people says that good content can improve the pagerank, but there have nice math function for this, so you can see which are the pagerank factors.

If you create autoblog that post news' feeds, you will see a lot of traffic, but after some period the domain will be deindexed - if there have placed adsense the domain will be deindexed very fast and you can lose some check.

Add your own content - I think that if you do it 50/50 your own/copy paste content, there will have not a problems.

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